It is desirable to convert analog electronic signals to digital form with an analog to digital converter, and a great many electronic circuits are known for this purpose. These analog to digital converters are known to have limitations on the speed at which they can operate in a given integrated circuit technology (i.e.—the feature size, process and material used to fabricate the integrated circuit) and therefore on the bandwidth of the analog signals that they can be used to convert. As these limits are approached, the converters are also known to consume more energy per sample, to have poorer manufacturing yield, and to have degraded accuracy.
In order to overcome this speed limitation, it is known to combine two or more analog to digital converters each acting as “subconverters” of a faster converter system. In the most common approach, the subconverters sample the input signal in a “round robin” fashion. As a special and well-known case, two subconverters may alternate taking samples so that the overall system has an effective sampling rate twice as high as that of the individual subconverters. One way of implementing this is to drive the sampling on each subconverter with a different clock, the two clocks being 180 degrees apart in phase. In the general case of N subconverters, each of the subconverters sample at phases 360/N degrees apart and the effective sampling rate is N times higher than the sampling rate of a single subconverter. These converter systems are often referred to as “multipath converters” or “N-path converters”.
It is also known to apply the multipath technique repeatedly, so that a subconverter is implemented as a system of yet slower sub-sub-converters.
Analog to digital converters are known to suffer from practical nonidealities, such as offset voltages, gain errors, bandwidth limitations, and various types of nonlinearities. In multipath converters these nonidealities also differ from one subconverter to another, and these “mismatches” create additional types of nonideality which cause additional difficulties. For example, some radio receiver systems are quite tolerant of a small gain error in analog to digital conversion, but much less tolerant of the gain varying from sample to sample. Further, the circuitry used to define sampling times is also subject to nonidealities, so that practical sampling phases may not be exactly 360/N degrees apart and this also causes difficulties in interpreting the data in many circumstances.
Prior art techniques for reducing the effects of nonidealities of ADCs include performing calibration of the analog to digital converters, including multipath analog to digital converters, by applying known signals, observing the resulting outputs, and making appropriate adjustments to the operation of the circuits or to the interpretation of their outputs. This prior art approach is often referred to as “off-line” calibration, because the requirement to apply a known signal interferes with the intended operation of the analog to digital converter in measuring unknown signals. Off-line calibration can be done as part of manufacture, or at the time of powering-up a system but in the practical case, where many errors “drift” with time, the quality of calibration will degrade over time.
Therefor, it is desirable, to perform “on-line” calibration, if possible, which is capable of tracking drifts in the errors of the ADCs. Algorithms suitable for on-line calibration can also generally be applied to known signals, and hence used for initial training if desired. A known two-path on-line calibration method was previously developed by the present inventor and others and is shown in U.S. Pat. No. 6,804,359 to Yu et al. The system taught in Yu can deal with offset and sample-phase timing errors of a two path ADC but, while the system taught in Yu can be useful, it does suffer from disadvantages in that it cannot deal with more than two ADC's and cannot address errors such as gain and bandwidth mismatch.
It is desired to provide correction circuitry operable to correct for mismatches and errors between N subconverters in multipath analog to digital converters.